Anode impedance and grid/LEM capacitance measurements Caspar Schloesser 1
Summary • Impedance measurements • 6x6x6 and 3x1x1 anodes • Different connectors and KEL cable • LEM-grid capacitance measurements • For different LAr levels and CRP positions • Comparison to • Theoretical value • Levelmeter readings 2
Probe Measurement setup • Impedance was measured with a Tektronix Anodes linked with PCB connector TDS 8000 Digital Sampling Oscilloscope for: • 3x1x1 anode module • 6x6x6 anode module • Strips at various positions and views Network analyzer • Anode + FEP/Kapton/PCB/KEL connectors • The 2 anode modules linked together • The 2 contacts of the probe were placed on the strip connection and the ground of the anode 3
Nothing connected • With nothing connected to the probe, only the impedance of the 50 Ω cable is measured • At the probe connection it goes to ∞ 50 Ω cable 4
6x6x6, 3x1x1 anodes center strip comparison • Impedance between 100 – 140 Ω for strips of both anode modules • Contacts from connector to strips have slightly different lengths → slightly different durations for signals to traverse entire copper track End of strips 50 Ω cable 5
Comparison of different strips for 6x6x6 anode • Contacts from connector to strips have slightly different lengths → slightly different durations for signals to traverse entire copper track Edge strip 6
6x6x6 anode + connector • Impedance not matched well for Kapton connector • Impedance matched for KEL ribbon cables 12.25 ns PCB KEL 1.6 ns 2.2 ns 2.7 ns Kapton 4.0 ns FEP 7
Signal speed in anode • Effective length of 1 anode strip: 50 cm + 80 * 2 * 1.5 mm = 74 cm (each strip passes 80 times below the perpendicular strips for 1 anode module) • Time it takes signal to cross 1 strip: 12.25 ns • Signal speed: 6.0e7 m/s = 0.20 * c 0 Taken from Pin-Jung Chiu 8
FEP PCB Kapton KEL Signal speed in connectors • PCB connector: 6.5 cm / 1.6 ns = 4.1e7 m/s = 0.14 * c 0 • FEP connector: 11 cm / 2.2 ns = 5.0e7 m/s = 0.17 * c 0 • Kapton connector: 7.5 cm / 2.7 ns = 2.8e7 m/s PCB = 0.09 * c 0 KEL • KEL cable: 25 cm / 4.0 ns = 6.3e7 m/s = 0.21 * c 0 Kapton FEP 9
Anodes chained together • Impedance matched for PCB connector within 15 Ω • Impedance also matched for KEL cable, connecting anode to SGFT flange Impedance matched within 15 Ω 10
LEM-grid capacitance measurements • Capacitance measured with Sourcetronic ST2831 LCR meter • Cs-Q mode LCR meter • At 200 kHz • Measured between grid connection on north flange and LEM up/down connections on SCFT HV flanges • All other LEM cables were disconnected, LEMs floating LEM HV connections Anode LEMs Grid 11
CRP level set at -0.5 mm • Measured on Thursday, 05.Oct.2017 at 14:24 • CRP level set at -0.5 mm through SPFT motors Capacitance LEM down / grid Average: 193.75 pF Capacitance LEM up / grid Average: 168.5 pF 12
CRP level set at -4.5 mm • Measured on Thursday, 05.Oct.2017 at 14:39 • CRP level lowered to -4.5 mm through SPFT motors Capacitance LEM down / grid Average: 235.25 pF Capacitance LEM up / grid Average: 204.33 pF 13
LAr removed and CRP level set to 0.0 mm • Measured on Thursday, 05.Oct.2017 at 15:04 • 3 mm LAr removed and CRP level raised to -4.5 mm through SPFT motors Capacitance LEM down / grid Average: 169.08 pF Capacitance LEM up / grid Average: 146.25 pF 14
LAr added and CRP level set to 0.0 mm • Measured on Friday, 06.Oct.2017 at 10:20 • 2 mm LAr added and CRP level set at 0.0 mm through SPFT motors Capacitance LEM down / grid Average: 165.75 pF Capacitance LEM up / grid Average: 142.92 pF 15
LAr added and CRP level set to -4.5 mm • Measured on Friday, 06.Oct.2017 at 11:58 • 2-3 mm LAr added and CRP level lowered to -4.5 mm through SPFT motors Capacitance LEM down / grid Average: 223.92 pF Capacitance LEM up / grid Average: 193.5 pF 16
Theoretical capacitance • Modelling the grid and LEMs as parallel plate capacitors, partially filled with a dielectric (LAr) • Capacitance between grid and LEM down given by: −1 1 1 𝐷 𝑠𝑗𝑒/𝑀𝐹𝑁𝑐𝑝𝑢𝑢𝑝𝑛 = + 𝜁 𝑠 (𝑀𝐵𝑠) × 𝜁 0 × 𝐵 𝜁 0 × 𝐵 𝑒 𝑀𝐵𝑠/𝑀𝐹𝑁 𝑒 𝑠𝑗𝑒/𝑀𝐵𝑠 17
Measurement deviations • Capacitance is dependent on LCR meter frequency → indication of inductive component • Stray capacitance from other components • Theoretical capacitance calculated for parallel plate model • LEMs and grid are not perfect even surfaces • Hard to remove effect of capacitance to ground • LCR meter gives different readings depending on which probe is placed on the grid 18
Comparison to levelmeters • Average of all levelmeters was taken over given time period and plotted against capacitance measurements • More measurements are needed to determine their exact relationship 19
Conclusion and next steps • Impedance has been measured for strips of the anode modules and different connectors • Impedance matches for KEL cables and PCB connector • 3L preamps have been set up in the clean room of Bldg. 185 and will be used to pulse the new anodes and connectors • LEM/grid capacitance measurements reflect the level of LAr between them • Deviation from theoretical values due to stray capacitance, inductive components, etc. • Capacitance can be used to determine liquid level after initial calibration • We will continue to measure capacitances as CRP is adjusted and/or level is altered to obtain the full relationship between capacitance and levelmeter reading 20
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